In general this type of rotating boring tool, as shown for example in FIG. 10A, 10B is provided at its tip end of a body 1 with main cutting blades 2a and 2b in two places of point symmetry with respect to the axis of the tool. In the body 1 two chip discharging grooves 3a, 3b (hereinafter referred to as the flutes) are formed continuously from the main cutting blades 2a and 2b. Tool sections within the range where the flutes 3a and 3b are formed are in point symmetry with respect to the axis of the tool at any point as shown in FIG. 10B. A metal cutting drill in general is of the same system as the above-mentioned tool notwithstanding a difference in the shape of the cutting blades at the tip end.
There is also a rotating boring tool provided with three flutes and the same number of cutting blades equally spaced at the tip end of the body. In this case also, the tool sections within the range in which the flutes are formed are arranged at a nearly equal center angle with respect to the axis of the tool at any point.
The rotating boring tool stated above has been considered to cut well while advancing straight on account of the equal, well-balanced distribution of cutting resistance to the cutting blades during boring. Actually, however, it has become clear that it is very difficult to equally distribute the cutting resistance to each cutting blade. That is, because of low-precision mounting of the rotating boring tool to the rotating spindle, or a machining inaccuracy of the tip end of the tool occurring in manufacturing the rotating boring tool, and further nonuniform internal structure or presence of directional properties in a work to be bored, the uniformity of cutting resistance is deteriorated during boring operation even if the rotating boring tool is mounted accurately and other conditions are in order.
Therefore since a component force of the cutting resistance, frictional resistance, etc. acts at the tip end of the rotating boring tool, at right angles with the axis of the tool, the straightforward advance of the rotating boring tool is disturbed, and it causes the rotating boring tool to swerve more or less during boring. Particularly if the effective length of the flutes empirically exceeds seven times longer than the tool diameter, boring accuracy will be lowered when the rotating boring tool is used in deep hole boring.
Also, there sometimes occurred such a trouble as sudden breakage of the rotating boring tool. In most cases, no cause of the breakage could be found; and it has been thought to the extent that it is the cause to break easily that the rotating boring tool is slender. The inventor et al of the present invention, however, have made researches of breakage of the rotating boring tool, finding out a cause of the breakage.
In the case of the rotating boring tool having the aforesaid two flutes, an arbitrary section of a portion in which the flutes are effectively formed is of the shape shown in FIG. 10B. It is, however, clear that this portion has lower rigidity against bending in the y direction than in the x direction. The flutes are spiral and the tool bores while turning. Therefore the direction of the component force stated above coincides with the y direction in the section of a certain portion of the range within a 1/2 lead (hereinafter termed the f1 portion) before the terminal end portion f0 of the flutes is reached. Consequently if the component force perpendicular to the axis of the tool acts at the tip end of the rotating boring tool, the tool will be warped at some point of the f1 portion, swerving during boring. That is, the rotating boring tool will be subjected to repetitive bending at a rapid cycle within the f1 portion during rotation for boring. This bending remains within an elastic limit; the repetitive boring operation will cause a fatigue to arise in the f1 portion, resulting in a breakage in the f1 portion of the rotating boring tool.
Such a problem can be prevented by increasing the rigidity of the f1 portion; as a method for increasing the rigidity, the flutes in the entire portion or in the f1 portion are made shallower to increase the axial width of the rotating boring tool or made narrower to increase the width of the entire section. According to these two methods, however, it is necessary to shallow or narrow the flutes to the extent that chips can not smoothly be discharged. Particularly these methods are unapplicable to a rotating boring tool used in a deep hole boring of wood or the like which produces a large amount of chips per revolution of the tool.
To solve the above-described problem, the present invention has an object to provide a long-life, hard-to-break rotating boring tool having greater rigidity in the f1 portion without adversely affecting smooth discharge of chips in order to assure unswerving boring during boring operation.